Push-pull amplifier



p 20, 1932 H. WHEELER ,87 ,7 3

PUSH-PULL AMPLIFIER Filed Aug. 1'31. 1929 'INVENTOR Mae/er ATTORNEYS Patented Sept. 20,1932 I I men) A. WHEELER,

push-pull circults.

UNITEDySTATES PATENT OFFICE OF GREAT NECK, NEW YOItK, ASSIGNOR TO HAZELTINE CORPO- BATION', A CORPORATION OF DELAWARE PUSH-PULL AMPLIFIER This invention relates to improvements in amplifiers of the'low frequency type adapted principally for amplifying currents in the audio frequency .range. More particularly, the invention is directed to improvements in thermionic amplifiers operating in so-called An object of the invention is to provide an amplifier of the type mentioned which will furnish substantially distortionless amplifi -cation without the necessity for utilizing a" biasin battery or an initial, negative biasing potential on the control circuits, or grids, of

the thermionic tubes.

A further object is to provide an amplifier of the type specified adapted to furnish a high degree-of substantially distortionless amplification.

A further object is to provide an amplifier of the typespecified capable of delivering relatively large' amounts of distortionless power output. 1

Alfurther object is to provide an amplifier of the type specified wherein the signaling current delivered to the output circuit is caused to flow in series thru the plate circuits of the tubes oppositely connected thereto,

such arrangement permitting the most. eificient delivery of distortionless power to the output circuit.

A further object is to provide a meter in the amplifier circuit which gives,a continuous indication of the signal intensity.

A further obj ectjs to provide an amplifier of the type s ecified havinga-relativelylow output impe' ance, as a conse uence ofutillzin no biasing potential out e grids of the tufies and further as a result thereof requirin a correspondingl lower. anode supply v0 tage than would ot erwise be required. Asa general proposition, one of the essen: tial re ufsites to the distortionless amplification 0 the signaling currents by means of thermionic amplifiers consists in applying to the grids of the thermionic tubes a sufiiciently large initial or permanent negative biasing potential to maintain them at all times ne 9;- tive relative to the corresponding catho es or filaments. For, if the incoming signal wave has a sufliciently great peakvalue to crease in voltage impressed between the grid and filament thereof as compared to that of the corresponding negative portion of the signaling wave. As a consequence of this, the amplified signaling wave will contain a flattened portion during each intervalthat .the grid is positive, which effect, of course,

is distortion;

It'is possible. by means of a well designed thermionic amplifier utilizing a sufliciently large permanent negative biasing potential on the grid,'to obtain substantially distortionless amplification, but such a result is attained only by sacrificing certain other desirable features. In the first place, the use of the negative biasing potential on the grid of .a tube increases the plate circuit impedance thereof by a corresponding amount since the presence of the negative potential on the grid impedes the flow of electrons from the filament to the plate, thus requiring a higher plate circuit voltage for a given plate current. The increased plate voltage required,

totgether with the added grid bias volta e, involves an added expense to the ampli er construction.

In order to partially overcome the above enumerated disadvantages accompanying the use of apermanent negative biasing potential on the grids of'the-tubes, the so-called push-pull amplifier .circuit was developed."

The push-pull amplifier, as frequently constructed, comprises in essence and input transformer having the secondary winding thereof connected between the" respective grids of a pair of similar thermionic tubes, an output transformer having the primary ,supp'l source to the midpoint of the output trans ormerprimary winding.

If now asine wave of slgnahng current 15 'winding thereof connected respectively beimpressed upon the input transformerof sucha push-pull amplifier, there will at each instant of time be signal voltage impressed in a positive sense between grid and filament 5 of one tube and an equal signal voltage impressed in the negative sense between grid and filament of the other tube, and since the voltages thus impressed produce additive effects in the output transformer secondary, it is ,thus insured that each positive half cycle of amplified signaling current will be identical in wave form with the corresponding negative half cycle. This, however does not necessarily mean that the amplified wave will be distortionless; it merely means that the even harmonics of the fundamental frequency will be eliminated. .Tlie odd harmonics may still be present in suificient magnitude to cause appreciable distortion.

In addition to the distortion effects noted with the push-pull amplifier as commonly constructed, any grid current in flowing thru the input transformer secondary winding will cause a considerable voltage drop therein which will reduce the net signaling voltage applied between the grid and filament of the tubes and in this way limit the amplification obtainable.

It is proposed in the present invention to improve the push-pull amplifier circuit as commonly arranged in such manner as to substantially eliminate the above noted objectionable effects which result from operating ;the same without an initial or a permanent negative biasing potential applied to the grids of the vacuum tubes. The improve-. ment is accomplished by serially interposing a high impedance element traversable by direct current, such as an inductance or a resistance, in each lead connecting the grids of the respective tubes to the input transformer secondary. Y The objects to be accomplished by such modification are to substantially prevent the flow -of current in the id circuits of the tubes and also to prevent t e grids being carried positive to an appreciable extent by the applied signaling waves.

Since with the above modification, current flowing in the grid circuit of either tube must traverse the high impedance element connect- .ed in the grid lead thereof, such impedance operates automatically to liniit the magnitude of current flow. Thus by making the impedcan be limited to such a degree as to produce negligible distortional or amplification limlting effects upon the amplifier operation.

By making the impedance elements serially I connected in the grid leads of large value rela-' Y This means that the tube willbe substantially ances sufiiciently high, the grid current flowinactive while pos'itivel biased by the impressed signaling wave, ut will strongly repeat the signal while negatively biased thereby since in the latter case the grid to filament impedance amounts to practically an open circuit across which the full signaling voltage is necessarily impressed. Due to the opposite manner .in which the tubes are connected to the input circuit, one tube will always'be charged negatively by the impressed signaling wave while the other tube is charged positively thereby, with the result that one tube will repeat positive portions and the other negative portions thereof in the output circuit. Further, the negatively charged tube will at each instant be responsible for substantially the entire signal repeated in the output circuit, with the positively charged tube contributing very little. This mode of operation is in contrast wit-h the operation of the push-pull amplifier as commonly constructed wherein the positively charged tube furnished the major portion of the output current.

Due to the fact that with the amplifier as disclosed herein only one tube at a time is active in delivering signaling current, it has been found that the commonly used mode of connecting the output circuit to the amplifier, namely, thru an output transformer having the primary winding tapped at the midpoint and connected thru a B battery to the filament, does not permit the maximum power output from the present circuit. Sinceonly one tube at a time is active the output transformer in such case acts as a three winding transformer having a single primary connected to the active plate, and a pair of windings connected respectively to the plate of the inactive tube and to' the output circuit. The dynamic plate circuit resistances of both tubes are thus connected effectively in parallel with the output circuit so that the inactive plate acts'as a dissipative shunt across the output circuit.

It is well known that in the operation of thermionic tubes the maximum power output consistent with good operation is obtained when the load impedance is substantially greater than the plate resistance. In the above case, the effective load impedance of 11 active plate and output circuit in parallel 1s always less than that of the active plate resistance.

In contrast with the above arrangementwherein the output circuit and the plate circuit resistance are effectively in parallel, it is possible tov so arrange the elements that the amplified signaling current flows in series thru the active andinactive plate resistances and thru the output circuit. With such an arrangement the above stated ideal power output condition can be realized to the extent that nearly the entire output from the active tube is delivered to'the output circuit and very little dissipated in the plate resistance of the inactive tube. Withsuch an arrangement a much smaller signal current W111 flow in the plate circuit of the inactive tube.

7 plate resistances of the active .and inactive.

tubes and thence thru the output circuit.

The series arrangement of the output circuit is inferior to the usual output transformer arrangement of thepush-pull amplifier as commonly constructed and using a high initial negative grid bias. The reasonls that with such an amplifier both tubes are'about equally active at each instant in delivering current tothe output circuit and the less active tube is the one with the higher instantaneous plate resistance.

Referring now to the drawing; Fig. 1 shows in circuit diagram form, an amplifier having the input circuit thereof arranged in accordance with the present invention. The output circuit is shown in ac- -35" cordance with the commonly used construction wherein the plate resistances of the amplifier tubesare effectivel'y'in parallel with the output circuit.

Fig. 2 shows a modification of Fig. 1 wherein a series arrangement is used for the output circuit.

' Fig. 3 shows a modification of Fig. 1 where in a special type of loud speaker unit is connected' directly in the anode circuits of the amplifier tubes with resultant advantages pointed out below.

The expression divided input section and divided output section as used in the specification and claims denotes the push-pull connection of the amplifier tubes in which the cathodes of the push-pull connected tubes are joined together and connected respectively to an intermediate point of the input circuit and output circuit, while the grids are connected to respective opposite outer terminals of the input circuit and the anodes are connected to i opposite outer terminals of the 'outputcircuit.

Referring toFig. 1, the amplifier comprises vacuum tubes V and V ,-each having grid-4 G, filament F and plateP electrodes. The grids of tubes V and V are connected respectively thru similar resistance elements R and R to the upper and lower terminals of a secondary winding S of transformer T the primary winding P of which is connected to a air of spa terminals 1;. The plates of tu es Vi and V are connected respectively to I the outputtransformer primary P is connected'by means of lead 2 thru a space supply sourceB tothe filaments of the tubes. The filaments F may be heated in any suitable manner, notshowng In the operation of the device, when signaling current is so applied to the input terminals I that the lower terminal of winding S is egative relative to the upper terminal, theig 'd of tube V will be carried negative and that of V positive by the signal voltage induced in the secondary winding. Grid current will flow in the space pathof tube V; from the filament to the grid of the tube and in series thru the resistance element R and tire upper'portion of the secondary winding S back to the filament. The resistance R being of large value relative to the filamentto-grid tube resistance, substantially all of the voltage drop due to the grid current'will be across the resistance and very little impressed upon the tube input as stated above, causing the tube V to weakly repeat the signal to the output circuit.

On the other hand, since the grid of tube V is carried negative by the signaling wave, the grid circuit of this tube ofiers practically an-dnfimte im edance connected in series with resistance 2 across the lower portion of the transformer seconda result that the full signalmg voltage induced in such portion is impressed between grid and filament of the tube thereby causing the signal to be strongly repeated. in the output circuit.

When" the impressed signaling voltage is such that the upper terminal of S is negative relative to the lower terminal, the operating conditionswill be reversed, the grid of tube V beingcarried negative 'and that of tube V, positive by the signaling current. Tube V will thus strongly repeat the signal to the ont- V put circuit, while the large voltage drop winding, with the across resistance element R due to the grid current flowing therethru will prevent tube V from bein active during such interval as was ex laine above. v v f Wit reference to the output circuit, it is obvious, of course, that during the interval that tube V is active, the signaling energy delivered thereby is dissipated in the output circuit and in the plate circuit resistance of tube V;.. The output circuit and the plate signaling current to and thence thru the entire circuit of tube V1 are connected effectively in parallel with the plate circuit of tube V2 with the resultant disadvantages pointed out above.

Referring now to Fig. 2, the series arrangement of the output circuit can be obtained by interposing a high impedance element, such as a choke coil L, in lead 2 containing the source of anode current B. The insertion of this im edance changes essentially the operation 0 the output circuit from that of the arrangement of Fig. 1.

When no signals are being received, the steady space current supply for tubes V and V flows thru the choke coil L. During reception of a signal, however, the space current will decrease in the'plate circuit of the active tube. A very slight corresponding decrease of current thru the inductance element L will set up a chokin action which will force substantiall all 0 the amplified ow in series thru the plate circuits of the active and inactive tubes primary winding P oftransformer T A choke coil L is preferable for interposition in lead 2 since it offers a relatively low resistance to the space current and hence permits the use of a correspondingly low space supply source B. It will be understood, however, that a -resistance element can be used for this pur ose or any type of high impedance traversahle by uni-directional current.

- In Fig. 2 the generalized impedance elements Z and'Z', replace the corresponding resistance elements R, and R of Fig. 1.:

' The amplifier shown in Fig.3 is similar to that shown in Fig. 1, with the exception that the anode circuits of tubes V and V contain the voice frequency actuating coils 5 and 6 respectively of a special type of loud speaker, known as a Farrand inductor dynamic speaker.- This speaker, which is especially adapted for use with'the type of amplifier disclosed herein, comprises the coneshaped acoustical diaphragm 7, a permanent horse shoe magnet 8 having aflixed to the respective poles thereof, oppositely positioned U shaped pole pieces 9 and 10. The pole pieces 9 and 10 are so dimensioned and positioned as to leave equal air gaps between the opposed legs thereof, in which space an armature 11 is suspended in such manner as to be free to vibrate in a per ndicular direction. A rod 12 connects the ower end ofarmature 11 to the central point of the cone 7, whereby the cone is vibrated in accordance with vertical movements of the armature. i

The coil 5 is placed about the upper leg of ole piece 10 and the coil 6 about the lower eg 0 pole piece 9. Also coil 5 is connected directly in the plate circuit of tubeV' and coil 6 directly in the plate circuit of tube V in such manner that the flux set up by the direct space current in each case aids the flux be pulled equally in opposite vertical direc tions by the steady flux threading the two air gaps between the pole pieces.

If, now, a signal wave be impressed upon the input terminals of the amplifier, the action of the amplifier is such that the current in signal coil 5 decreases when the grid of tube V is negative, and at the same time the current in signal coil 6 remains constant at the normal direct current value, or increases very little, because the -grid of tube V is positive. Likewise, when the grid of tube V;

. is positive, the current in signal coil 5 is practically at the normal direct current value and the current in signal coil 6 is decreased. From the foregoing it is seen that the signal coils are alternately operative and furthermore the coil which is inoperative exerts no influence upon the coil which is operative. This gives the result that the inactive tube of low plate impedance is not effectively a shunt across the outputof the active tube, as would be true if a single output transformer were utilized.

minals of an output transformer, as for'ex ample, to terminals 0 of Fig. 2, the lower The principal advantage of connecting the frequencies would in general bediscriminated against. Another advantage of the arrangement of Fig. 3 lies in the eliminatlon of the output transformer T which, of course, represents an economy in amplifier construction.

The greatest distortionless power output 'is secured when coils 5 and 6 are practically without mutual magnetic coupling and when the effective impedance of each vcoil is approximately three times the output impedance of the vacuum tubes individually 'connected thereto.

Referring back for a moment to Fig. 1, the

resistances R 'and, R should be made equal to the corresponding grid to filament resistances of the tubes for amplifying impressed signals of relatively small amplitudes. It has been found that such proportioning of the elements mentioned improves the distortionless amplification obtained. In general a value of .1 megohn for R and R is proper for this purpose. I

With the series arrangement for the output circuit as shown in Fig. 2-, the circuit has been found to operate'most efliciently when the apparent load impedance connected between the anodes of the tubes equals approxicuits with a pair of vacuum tubes connected in opposition therebetween, each of said tubes having a grid and a cathode, said grids being at the same potential as said cathodes in the absence of signaling waves insaid input circuit, high resistance means interposed between said grids and said input circuit for rendering one tube unresponsive to positive portions and the other tube unresponsive to negative portions ofelectric'waves in said input circuit.

2.'Electric wave repeating apparatus for,

prising divided input and divided output cir-' substantial entirety upon thereleme'nt whose input electrode receives a negative potential therefrom, whereby said latter elementalong repeats the signal, and means connecting said elements in opposition to an-output circuit. -5. Electric wave repeating apparatus for providing a high degree of substantially distortionless amplification in the absence of biasing potentials in the control circuits thereof, comprising divided input and divided output circuits with a pair of vacuum tubes connected in opposition therebetween,

each of said tubes having a cathode, anode' and grid, said cathodes being connected together, a resistance interposed between the grid of each said tube and said input circuit, whereby signaling waves present in said inproviding a high degree of substantially disput circuit are ,impressed'at each instant in biasing potentials in the control circuits thereof, comprising divided input and divided output circuits with a pair of vacuum tubes each including grid and cathode connected in opposition between said circuits, resistance means interposed between said grids and said input circuit to substantially prevent the application of a voltage in a positive sense from said cathodes to said grids whereby signal waves present in said input circuit are impressed in substantial entirety upon the tube which receives a negative potential therefrom, whereby saidlatter tubesalone repeats the signal. L

3. in an electric wave repeating apparatus for providing a high degree of substantially distortionless amplification in the absence of biasing potentials in the control circuits thereof, the combination of two repeating ele ments each comprising a cathode, an anode and an input electrode, the cathodes ofb'oth elements being connected together, an input repeating coil the secondary winding of which is connected at an intermediate point to said cathodes and at its opposite terminals to said input electrodes respectively through resistances adapted to cause signaling waves flowing in said coil to be impressedat each instant in substantial entirety upon the element .whose input electrode receives a nega tive potential therefrom, whereby said latter element alone repeats the signal.

4. In an electric wave repeating apparatus for providing a high degree of substantially distortionless amplification in the absence of biasing potentials -in the control circuits thereof, the combination of two repeating elements each comprising a cathode, an anode and an input electrode, the cathodes of both elements being connected together, an input repeating coil the secondary winding of which is connected at an intermediate point to said cathodes andcat its opposite terminals thru impedance elements to said input' electrodes respectively, whereby signaling waves present in said input coil are impressed in tortionless amplification in the absence ofsubstantial entirety upon the tube whose grid receives a negative potential therefrom, 'so that said latter tube alone repeats thesignal, and impedance means interposed in said output circuit for causing repeated electric waves to flow in series thru the anode resistance of both s id tubes and said output circuit.

6. ctric wave repeating apparatus for providing a high degree of substantially distortionless amplification in the absence of biasing potentials in the control circuits "thereof, comprising a pair of vacuum tubes each having anode, cathode and grid electrodes, a divided input circuit'connected at an intermediate point to said cathodes andat Topposite outer terminals thru impedances to said grids respectively, whereby signal waves present in said input circuit are impressed in substantial entirety upon the tube whose grid momentarily receives a negative potential therefrom whereby said latter tube alone repeats the signal and a divided output circuit connected at an intermediate point thrii an.

and-the anode-cathode space paths of said tubes.

7 Electric wave repeating apparatus for providing a high degree of substantially distrodes, an input transformerthe secondary winding of which is connected at an intermediate point to said cathodes and at its opposite terminals thru resistances to said grids respectively, whereby signal waves present in said input transformer are impressed in sub- :stantial entirety upon the tube whose grid momentarily receives a negative potential therefrom, so that said latter tube alone rer peats thesignal, andan output transformer v the primary winding ofwhich is connected thru an inductance to said cathodes and at its tortionless amplification in the absen e of U resent in said input circuit are impressed in substantial entirety upon the tube whose grid momentarily receives a negative potential therefrom so that said latter tube alonerepeats the signal, of a loud speaker unit in-{ cluding means forproducing a permanent magnetic flux adapted to maintain in dynamic equilibrium an armature afiixed to an acoustical unit, a pair of signal current coils for actuating said armature situated respectively at op osite ends thereof, means so connecting said coils in the anode circuits of said tubes respectively that the fluxes due to the space currents flowing therein in each case aid said permanent flux, whereas signal waves repeated by said tubes produce dis tortionless resultant actuating effects upon said armature.

9.. Electric wave repeating apparatus for providing a high degree of substantially dis-- tortionless amplification in the'absence of biasin potentials in the control circuits thereo comprising'divided input and di- 4 vided output circuits with a pair of electron discharge devices each including anode, cathode and grid connected in opposition therebetween, and high resistance means interposed between said grids and said input circuit adapted to render at each instant substantially unresponsive to a signaling wave impressed upon said input circuit the said discharge device which receives a positive potential'therefrom, whereby signal waves resent in said input circuit are impressed in substantial entirety upon the device whose grid momentarily recelves a negative potential therefrom, whereby said latter tube alone is effective to repeat the signal.

10. Electric wave repeating apparatus for providing a high degree of substantially disv tortionless amplification in the absence of biasing potentials in the control circuits thereof, comprising divided input and divided output circuits with a pair of electron discharge devices each including anode, cathode and grid connected in opposition therebetween, and resistance of large magnitude relative to the lowest cathode-to-grid resistanceiof said devices during'operation interposed between each saidgrid and said input thereof, comprising in substantial entirety upon the devicewhose grid momentarily receives a ne 'ative tential therefrom whereby said fatter tube alone repeats the signal.

'11. A thermionic amplifier adapted to provide high degrees of substantially distortionless amplification in the absence of negative biasing potentials in the control circuits of the thermionic elements, comprising divided input and divided output circuits with a pair of thermionic tubes each including anode, cathode and grid connected in opposition therebetween and a resistance of large magnitude relative to the lowest cathode-togrid resistance of said devices during operation interposed between each grid and said input circuit for causin a signalingwave impressed thereupon to be repeated at each instant in substantial entirety :by the saiddevice which receives a negative potential therefrom. l

12. Electric wave repeating apparatus for a providing a high degree of substantially distortionless amplification in the absence of biasing potentlals in the control circuits divided input and divided outputv circuits with a pair of-electron discharge devices each including anode, cathode and grid connected in opposition. therebetween, high resistance means inter posed between said grids and said input circuit adapted to render at each instant sub stantially unresponsive to a slgnaling wa ve impressed upon said input circuit the discharge device which receives a positive potential therefrom, and addition means'associated with said output circuitadapted to cause the repeated signaling wave to flow in series through the anode circuits of both said devices.

13. Electric wave repeating apparatus for providing a high degree of substantlally distortionless amplification in the absence of biasing potentials in the control circults thereof, comprising divided input and di vided output circuitswith a pair .of electron discharge devices each including an ode, cathode and grid connected in opposition therebetween, a high resistance interpo'sed between each said-grid and said input circuit adapted to render at each 1nstant substantially unresponsive to a signaling wave impressed upon said input circuit the discharge device which receives a positive potential therefrom, whereby signals present in said input circuit are repeat ed only by the device whose grid momentarily receives a negative potential therefrom, and inductive reactance included in the space latter tube alone repeats the signal, of a loud current supply circuits of said discharge derepeated at each instant in substantial ent1 ety by the said device which receives a negative potential therefrom.- v

. 15. Electric wave repeating apparatus for providing a high degree of substantially distortionless amplification in the absence of biasing potentials in the control clrcuits thereof, comprising in combination a pair of electron discharge devices each having anode, cathode and grid, an input circuit connected at an intermediate point thereof to said cathodes in common and at its opposite terminals to said grids respectively through means adapted to cause a signaling wave impressed upon said input circuit to be repeated at each instant in substantial entirety by the said device which receives a negativepotential therefrom, and an output circuit connected at its outer terminals to said anodesrespectively and at an intermediate point thereof to said cathodes through space current supply means and an impedance adapted to cause the repeated waves to flow in series through the space paths of both said devices.

16. The combination with a thermionic am plifier for providing a high degree of substantially distortionless amplification in the absence of biasing potentials in the control produce additive distortionless resultant efects upon said armature. 17. Electric wave repeating apparatus comprising divided input and divided output circuits with a pair of vacuum tubes each having an anode, cathode and grid, connected in opposition therebetween, said input circuit being connected at an intermediate point to said cathodes in common and at opposite outer terminals through high impedances to said grids respectively, whereby signal waves present in said input circuit are impressed in substantial entirety upon the tube whose grid momentarily receives a negative potential therefrom whereby said latter tube is alone effective to repeat the signal.

HAROLD A. W EELER.

In testimony whereof I afiix m signature. a

circuits thereof, comprising a pair of vacuum tubes each having anode, cathode and grld, and a divided input circuit connected at an intermediate pomt to said cathodes.

and'at opposite outer terminals through re- 7 sistances to said grids respectively, whereby signal waves present in said input circuit are impressed in substantial entirety upon the tube whose grid momentarily receives a negative potential therefrom so that said speaker unit including means for producing a permanent magnetic flux adapted to maintam 111 dynamic equilibrium an armature afof, means so connecting said coils in the anode circuits of said tubes respectively that signal waves repeated-by said amplifier tubes 

